1. Field of the Invention
The present invention relates to an image forming apparatus and a process cartridge including multiple image bearing members provided in tandem in the image forming apparatus, and an image forming method using multiple image bearing members provided in tandem.
2. Description of the Background
An electrophotographic image forming apparatus generally includes a photoreceptor that bears electrostatic latent images. A surface of the photoreceptor is charged by electric discharge, and the charged surface is then exposed to light containing image information to form an electrostatic latent image thereon. The electrostatic latent image is supplied with toner particles to be developed into a toner image that is visible. The toner image is transferred from the photoreceptor onto a recording material, and finally fixed thereon.
After transferring the toner image from the photoreceptor, some toner particles may remain on the photoreceptor. Such residual toner particles are generally removed by a cleaner so as not to adversely affect subsequent image forming operations. The cleaner may be a blade-shaped member comprised of an elastic material, such as rubber, contacted against a surface of the photoreceptor, for example.
In accordance with recent demand for higher image quality, toner particles are required to be much smaller and more spherical. Generally, small toner particles have an advantage in dot reproducibility, and spherical toner particles have an advantage in developability and transferability.
However, such small and spherical toner particles are difficult to manufacture through widely used conventional toner manufacturing processes including steps of kneading raw materials and pulverizing the kneaded raw material mixture into particles. On the other hand, toner manufacturing processes using polymerization reactions, such as suspension polymerization, emulsion polymerization, or dispersion polymerization, advantageously manufacture small and spherical toner particles. Such toner particles manufactured through polymerization processes are put into practical use recently.
However, small and spherical toner particles cause some problems when remaining on the photoreceptor after image transfer.
The first problem is that such small and spherical toner particles are difficult to remove from the photoreceptor using a blade-shaped member (hereinafter “a cleaning blade”). When the cleaning blade slidably contacts a surface of the photoreceptor to remove residual toner particles, the photoreceptor-contacting edge of the cleaning blade deforms due to frictional resistance between the cleaning blade and the photoreceptor, thus generating a tiny space or gap between the cleaning blade and the photoreceptor. Smaller toner particles are more likely to get into the tiny space, and more spherical toner particles are more likely to roll within the tiny space and pass through the cleaning blade. The more toner particles passing through the cleaning blade, the more the resulting image quality deteriorates.
The second problem is that release agents and fluidizing agents, included in the toner particles passed through the cleaning blade or remaining on the photoreceptor, are likely to gradually adhere to the surface of the photoreceptor, forming a thin film thereon. This phenomenon is hereinafter referred to as filming. Filming generally causes abnormal images such as solid images with white spots.
In attempting to satisfactorily remove small and spherical toner particles from photoreceptor, Japanese Patent Application Publication No. 2002-287567 (JP-2002-287567-A) proposes to decrease the surface friction coefficient of a photoreceptor by applying a lubricant (e.g., a metal soap of a fatty acid) to its surface to form a thin layer of the lubricant thereon.
Meanwhile, image forming apparatuses employing an intermediate transfer member are widely used recently. In such an image forming apparatus, multiple different-color toner images are sequentially formed on multiple photoreceptors, and then sequentially transferred onto an intermediate transfer member. This process is called a primary transfer process. In the primary transfer process, the different-color toner images are superimposed on one another on the intermediate transfer member, forming a composite full-color toner image. The composite full-color toner image is finally transferred onto a recording material. This process is called a secondary transfer process.
In both the primary and secondary transfer processes, some toner particles in the toner images may not be transferred, and therefore the resulting toner image on a recording material may have local defects. When a solid image has such defects, each defect may occupy a considerably large area. When a line image has defects, the line may be interrupted by the defects.
The composite full-color toner image comprised of four color toners is more likely to cause such defects than monochromatic images. The first reason for this is that the composite full-color toner image has considerable depth or thickness, and moreover, repeating the primary transfer process four times generates considerable non-Coulomb mechanical adhesive forces, other than electrostatic forces such as van der Waals force, between the toner and the photoreceptor or the intermediate transfer member. The second reason is that the adhesive force between the intermediate transfer member and the toner increases along with formation of an undesired film of the toner (i.e., filming).
In attempting to avoid production of such defects, JP-2000-162881-A proposes to apply an optimal amount of a lubricant to the surfaces of a photoreceptor and an intermediate transfer member to reduce the adhesive force between a toner and the photoreceptor or the intermediate transfer member. Although a successful approach, in a case in which multiple photoreceptors are arranged in tandem, the lubricant applied to the intermediate transfer member may be further retransferred onto the extreme upstream photoreceptor, supplying the extreme upstream photoreceptor with an excessive amount of lubricant.
The excessive amount of lubricant on the photoreceptor may excessively decrease the surface friction coefficient, resulting in too small an adhesive force between the photoreceptor and the toner. Thus, the toner cannot reliably adhere to an electrostatic latent image on the photoreceptor, resulting in a toner image with defects and a low density.
Additionally, there is another concern that the excessive amount of lubricant on the photoreceptor may contaminate the image forming apparatus charging roller, thereby producing images of uneven density.